There's a well studied area of psychology around the "bystander effect" and diffusion of responsibility. Broadly speaking, each bystander might have looked to the others around him to step in (of course resulting in no one stepping in).
If I wasn't armed in this situation, I'd probably wade in. Not with the idea of winning (I'm only 5' 9" and about 175 pounds) but with inspiring the rest of the crowd to intervene.
This has been shown to be effective, but not foolproof. And it's certainly personally risky.